QWIPs are known (see, for instance, U.S. Pat. Nos. 4,894,526 and 5,023,685), and are very promising devices, especially in array form. However, QWIPs have not yet found widespread commercial use, due at least in part to the relatively low optical coupling of conventional devices, which results in relatively low responsivity and detectivity of the devices. It thus would be highly desirable to have available QWIPs with improved optical coupling. This application discloses such QWIPs.
For reasons that are well known to those skilled in the art, prior art QWIPs comprise a feature or features that insures that the optical electric field in the quantum well (QW) region has a component that is normal to the plane of the QWs. See, for instance, G. Sarusi et al., Applied Physics Letters, Vol. 64(8), p. 960 (Feb. 21, 1994), incorporated herein by reference. Thus, QWIP arrays conventionally comprise 1-dimensional or 2-dimensional gratings to provide this electric field component and to enhance optical coupling by causing the effective radiation to pass more than once through the QW region. However, the number of passes obtainable by means of even a 2-dimensional grating is small, namely two. See FIG. 1a of Sarusi et al. (op. cit.), and discussion pertaining thereto. See also J. Y. Andersson, Applied Physics Letters, Vol. 58, p. 2264 (1991).
It is known that the external quantum efficiency of LEDs can be substantially increased by texturing of the surface of the LED, such that angular randomization of the radiation is achieved by strong surface scattering. See I. Schnitzer et al., Applied Physics Letters, Vol. 63(16), p. 2174 (Oct. 18, 1993).
It is also known that the performance of a solar cell can be enhanced by provision of a textured surface that results in substantial randomization of the optical radiation. See E. Yablonovitch et al., IEEE Transactions on Electron Devices, Vol. ED-29 (2), p. 300 (1982).